Maths Olympiad Prep

Library / /653 of 860

Combinatorics Difficulty 5.3 AIME, harder Find the answer

A subset SS of the nonnegative integers is called supported if it contains 0, and k+8,k+9Sk+8, k+9 \in S for all kSk \in S. How many supported sets are there?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Note that every supported set SS contains 0,8,9,16,17,18,2427,3236,40450,8,9,16,17,18,24-27,32-36,40-45, 48-54, and all n55n \geq 55. Now define Sˉ:=Z+\S\bar{S}:=\mathbb{Z}^{+} \backslash S, which is a subset of {17,1015,1923,2831,37,38,39,46,47,55}\{1-7,10-15,19-23,28-31,37,38,39,46,47,55\} satisfying the opposite property that kSˉk8,k9Sˉk \in \bar{S} \Longrightarrow k-8, k-9 \in \bar{S}. Consider the above arrangement after removing the numbers not in Sˉ\bar{S}. The condition that SS be supported ensures that sets Sˉ\bar{S} are in bijective correspondence with paths from (0,0)(0,0) to (16,0)(16,0) consisting of discrete steps of 1,1\langle 1,1\rangle and 1,1\langle 1,-1\rangle and lying above the xx-axis: from the modified version of the above diagram, a unique path passes through the top items left in each column. The number of such paths is the 8th Catalan number, so the answer is C8=18+1(828)=128709=1430C_{8}=\frac{1}{8+1}\binom{8 \cdot 2}{8}=\frac{12870}{9}=1430. (Incidentally, 16 choose 8 was computed in an earlier problem.) Without the explicit formula for Catalan numbers, the answer can be computed recursively by filling in the number of ways a path can reach (16,0)(16,0) from each position in the figure. One works right to left, obtaining the following: One can exploit symmetry and, having determined the middle column, sum the squares: 12+72+202+282+142=14301^{2}+7^{2}+20^{2}+28^{2}+14^{2}=1430

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.